In telephony, use of both two-wire and four-wire modes for connecting distant-calling stations gives rise to the well-known problem of circuit echo. The source of most echoes are imperfectly balanced hybrid circuits at the two- to four-wire junctures.
Circuit echo is normally not detected by a subscriber if the circuit is short-distance. In this case, the echo signal cannot readily be discerned because it is almost coincident in time with the listener's side-tone level. As the two-way path becomes longer, however, the delay increases until the echo can be heard quite distinctly even when greatly attenuated. The problem is particularly present in circuits which includes a communication satellite, where the round-trip delay is nearly 0.6 second. A communications circuit with two satellite links would, of course, have double this delay.
Accordingly, echo suppressors are routinely used in telephone transmission paths where circuit echo can be expected. Presently, the type of echo suppressor most widely used is a voice switch that recognizes the presence of speech in, say, the incoming direction, and in response inserts a large loss in the outgoing direction. This greatly attenuates any echo signal that may leak across the hybrid but at the same time attenuates speech signals transmitted in the outgoing direction. Consequently a break-in mode is provided that permits the receiving party to override a stream of incoming speech. However, break-in is sometimes difficult to achieve. In the break-in process, moreover, speech is sometimes multilated; and during break-in, echo may be present.
One scheme for eliminating echo without impeding the free flow of conversion in both directions is disclosed in M. M. Sondhi U.S. Pat. No. 3,499,999 assigned to applicant's assignee. Here, signals incoming to a hybrid are supplied to a plurality of filter networks adjusted to develop a set of impulse responses. A linear combination of these responses approximates the typical echo impulse response. These linear transformations of the input signal are selectively adjusted in gain by a differential outgoing signal. This signal then is subtracted from signals in the outgoing circuit to render it relatively echo-free. A more generalized scheme practiced in accordance with the foregoing is disclosed in U.S. Pat. No. 3,500,000 of J. L. Kelly, Jr. et al. assigned to applicant's assignee. Both approaches enable double-talking to take place while the echo is being canceled.
An alternative to these schemes is disclosed in the U.S. Pat. No. 3,585,311 of D. A. Berkley and O. M. M. Mitchell. That invention contemplates dividing the bandwidth of the voice-frequency channel into several contiguous, subbands by passing the incoming signal through a bank of bandpass filters. The output of each filter is then center-clipped. Clipping distortion is removed by filtering the clipped signals in a second set of filters and combining their outputs for transmission. The clipping levels are fixed to close down each passband a large fraction of the time, assuring suppression of the relatively weaker (by 6 dB to 30 dB) power of the echo signals. Mentioned also were parameter-responsive clipping levels.
The present invention is directed to parameter-responsive clipping levels in the processor of U.S. Pat. No. 3,585,311.
The principal inventive object is to operate upon an echo signal present in a transmission path in such a way as to remove the echo signal and to leave unaffected those portions of the transmission path bandwidth that are relatively low in echo signal energy content.
A broad object of the invention is to permit a double-talk to occur in a long-distance telephone circuit, while at the same time reducing or masking the echo signals.
A related inventive object is to eliminate the need for monitoring equipment to detect occurrence of double-talk.
A further object of the invention is to avoid mutilation of speech attendant the suppression of echo during a double-talk situation on a long-distance telephone circuit.